The present invention relates to a buoy structure for detecting fishing grounds and more particularly a buoy that detects the fishing grounds therebelow by means of ultrasonic wave transmitting transducer and ultrasonic wave receiving transducer, by means of the time required for the sound wave echo detected and through the control of an PC board to produce an audible acoustic frequency, the signal of said acoustic frequency is then transmitted by a FM transmitter through a FM antenna of a mark rod and the signal thus transmitted is then received by the fishermen from a receiver through FM antenna; by means of the change of the frequency received to determine the situation of fishing grounds and to take up the fishing rod or from the display of the receiver to see the distribution of fishing grounds for fruitful fishing.